release.nix: Swap pythonVersion/system namespace
release.nix: Swap pythonVersion/system namespace

Previously we had jobsets like python27.x86_64-linux.hetzner, which
resultis in weird sorting in case we just like to see whether a build
succeeds for a particular Python version.

Now it's x86_64-linux.python27.hetzner, which is much better.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

RedMoonStudios/hetzner